This invention relates to electrodes, an alkaline secondary battery, and a method for manufacturing an alkaline secondary battery.
As a positive electrode to be employed for an alkaline secondary battery, a sintered type positive electrode has been employed. This sintered type positive electrode has been manufactured by a process wherein nickel particles are attached to a two-dimensional substrate such as a punched steel body or a nickel mesh and then sintered to obtain a porous plate having openings of several tens microns in pore size, which is then impregnated with an aqueous solution of a nickel salt and subjected to an alkaline treatment thereby to turn the nickel salt impregnated in the porous body into nickel hydroxide.
However, the manufacture of the sintered type positive electrode involves complicated procedures for impregnating a nickel salt and an alkaline treatment. In order to impregnate the substrate with a predetermined amount of an active material, the aforementioned procedures are required to be repeated 4 to 10 times. As a result, the manufacturing cost would be increased by such complicated procedures. Moreover, since the sintered type positive electrode to be obtained by the aforementioned procedures is accompanied with a problem that if the porosity thereof exceeds over 80%, it is difficult to assure the mechanical strength thereof, it is impossible to further increase the filling ratio of the active material.
Under the circumstances, it is now studied to manufacture a positive electrode by a different method wherein nickel hydroxide particles are mixed with a conductive material, a binder and water to prepare a paste, which is then filled into a metallic porous body having a three-dimensional structure such as a sponge-like porous metallic body having an average porosity of 95% or more and an average pore diameter of several tens to several hundreds microns, and a metallic fiber mat. The positive electrode manufactured in this manner is called a non-sintered type positive electrode (or a pasted type positive electrode) in contrast to the aforementioned sintered type positive electrode. This a paste type positive electrode is advantageous in that the porosity and average pore size of the metallic porous body are relatively large as compared with those of the aforementioned sintered type positive electrode, so that an active material can be more easily filled into the substrate and the filling ratio of an active material can be increased.
Accordingly, it has been possible with the employment of this paste type positive electrode and improvements on the quality of this paste type positive electrode to further increase the capacity of alkaline secondary battery. By the way, due to an increasing propagation and high performance of potable electric equipments in recent years, it is still demanded to further improve the density of capacity of a secondary battery.
It is imperative in order to meet such a demand to increase the positive electrode or the negative electrode. In the case of a nickel-hydrogen secondary battery, which is one of an alkaline secondary battery, the electrode which regulates the capacity of the battery is generally constituted by a positive electrode. Therefore, it is imperative in order to increase the capacity of battery to increase the ratio of an active material for the positive electrode. By the way, the nickel-hydrogen secondary battery is generally constructed in such a manner that an electrode group comprising a paste type positive electrode containing nickel hydroxide as an active material, a paste type negative electrode containing a hydrogen absorbing alloy, and a separator interposed between the positive electrode and the negative electrode is housed together with an alkali electrolyte in a case (or vessel). There is a limitation on the volume of the electrode group to be accommodated in the case. Therefore, as a means for increasing the quantity of an active material, it is conceivable to decrease the ratio in capacity between the positive electrode and the negative electrode, i.e. to decrease the capacity of the negative electrode, or to make the thickness of the separator thinner, thereby decreasing the volume of the electrode group. In this case, whichever method it is adopted, some decrease in quantity of alkali electrolyte in relative to a capacity of the positive electrode cannot be avoided. If the quantity of alkali electrolyte in relative to the capacity is decreased, the inner resistance would be increased to lower the operating voltage, thus raising the problems that the discharge capacity is lowered and the utilization of the positive electrode would be lowered.
Meanwhile, Japanese Patent Unexamined Publication H/2-30061 discloses a positive electrode containing nickel hydroxide particles to which zinc is added. Japanese Patent Unexamined Publication H/3-77273 discloses a positive electrode containing nickel hydroxide particles whose surfaces are adhered with zinc hydroxide particles. However; it is still difficult to avoid the lowering of the utilization due to an increase in capacity of the battery.